Occurrence of initiation factor 2 in the postribosomal fraction and identification of an initiation inhibitor as elongation factor G

Expression, purification, and mechanistic studies of bovine mitochondrial translational initiation factor 2

A complete cDNA clone encoding bovine mitochondrial translational initiation factor 2 (IF-2mt) has been obtained. The regions of the cDNA corresponding to mature IF-2mt and several of its functional domains have been expressed in Escherichia coli as histidine-tagged proteins. The precursor (approximately 90 kDa) and mature (approximately 85 kDa) forms of IF-2mt are toxic to E. coli and can only be expressed at low levels. Shorter forms of this factor (approximately 80 and approximately 72 kDa) are also found during the expression of mature IF-2mt. The various forms of IF-2mt can be separated by high performance liquid chromatography. All of these forms are active in promoting the GTP-dependent binding of formyl-Met-tRNA to the small subunit of either E. coli or bovine mitochondrial ribosomes. IF-2mt can bind to mitochondrial ribosomes in the absence of GTP, initiator tRNA, or messenger RNA. The presence of GTP stimulates IF-2mt binding to ribosomes about 3-fold. IF-2mt interacts only weakly with GTP or with the initiator tRNA in the absence of ribosomes. Molecular dissection of IF-2mt shows that a long deletion (approximately 150 amino acid residues) from the NH2-terminal region does not affect its activity in vitro. The COOH domain of IF-2mt (amino acid residues 332-727) can bind to ribosomes even though it does not promote initiator-tRNA binding.

Initiation complex formation on Euglena chloroplast 30S subunits in the presence of natural mRNAs

An in vitro system has been developed that allows the formation of translation initiation complexes with Euglena chloroplast 30S ribosomal subunits and natural mRNAs. For these experiments two regions of the Euglena chloroplast genome have been cloned behind the T7 transcriptional promoter and the corresponding RNAs synthesized in vitro. These mRNAs are capable of forming initiation complexes with chloroplast 30S subunits in the presence of fMet-tRNA and E. coli initiation factors. Deletion of the normal translation start site results in a message that is no longer recognized by the chloroplast subunits suggesting that the correct AUG initiation codon on the mRNA is being selected by the small ribosomal subunit. Initiation complex formation with the chloroplast 30S subunits is specific for chloroplast mRNAs and mRNA from the phage MS2 is not active in this system.

Cooperative effects of initiation factors and fMet-tRNA in the formation of the 40-S initiation complex

In this paper the mode of action of IF-1 in 40-S initiation complex formation was studied with MS2 RNA as messenger. Using initiation factors IF-2 and IF-3 labeled in vitro it appeared that IF-1 did not influence the binding of these factors in the absence of fMet-tRNA. However, in the presence of fMet-tRNA it was found that the enhancement of the fMet-tRNA binding by IF-1 was accompanied with an equimolar increase in binding of IF-2. Moreover, it appeared that also in absence of IF-1, fMet-tRNA binding is coupled with an equimolar enhancement of the IF-2 binding, which suggests the existence of a preribosomal complex between IF-2 and fMet-tRNA. The apparent Km values for both the binding of fMet-tRNA and IF-2 to 30-S subunits were determined and appeared to be equal, which makes a functioning of such a preribosomal complex in protein initiation very likely. The participation of GTP in this complex will be discussed. Functions of IF-1 in dissociation and recycling of IF-2, described by others, and the stimulation on the 30-S subunit level might well be explained as pleiotropic effects of one basic action of IF-1, i.e. a conformational change of 30-S subunits.

Identification of a soluble protein that stimulates peptide bond synthesis

A soluble protein factor was isolated, free of elongation factor (EF)-T and EF-G, based on its ability to stimulate the synthesis of peptide bonds using ribosomal bound 70S-AUG-N-formyl-[35S]methionyl-tRNA complex and added puromycin as substrates. Over 90% of this activity was found in the ribosome-free cytoplasm of Escherichia coli extracts. Otherfeatures such as molecular weight, purification properties, and catalytic activities distinguish this factor from ribosomal proteins and known activators of translation. The factor requires all components needed for peptide bond synthesis and is inhibited by antibiotics known to specifically block the peptidyl transferase activity of ribosomes. The factor increases the binding affinity of the ribosome for the aminoacyl-tRNA analog puromycin about 10-fold. We suggest that this extraribosomal factor modulates the intrinsic activity of ribosomes to catalyze peptide-bond synthesis, and regard it as a new factor required for peptide chain elongation, which we call EF-P.